(Not Applicable)
1. Technical Field
This invention relates to mass spectrometers, and more particularly to a submillimeter ion trap for mass spectrometric chemical analysis.
2. Description of the Related Art
Microfabricated devices for liquid-phase analysis have attracted much interest because of their ability to handle small quantities of sample and reagents, measurement speed and reproducibility, and the possibility of integration of several analytical operations on a monolithic substrate. Although the application of microfabricated devices to vapor-phase analysis was first demonstrated 20 years ago, further application of these devices has not been prolific due primarily to poor performance because of mass transfer issues. However, some low pressure analytical techniques, such as mass spectrometry, should be possible with microfabricated instrumentation. Recent reports of microfabricated electrospray ion sources for mass spectrometry make the possibility of miniature ion trap spectrometers especially attractive.
Ion traps of millimeter size and smaller have been used for storage and isolation of ions for optical spectroscopy, though not for mass spectrometry. The principal requirement for ion trap geometry is the presence of a quadrupole component of the radio frequency (RF) electric field. Conventional ion trap electrode constructions include hyperbolic electrodes, a sandwich of planar electrodes, and a single ring electrode. For more information concerning ion trap mass spectrometry, the three-volume treatise entitled: xe2x80x9cPractical Aspects of Ion Trap Mass Spectrometryxe2x80x9d by Raymond E. March et al. may be considered, and is incorporated herein by reference.
The smallest known quadrupole ion trap that has been evaluated for mass analysis or for isolation of ions of a narrow mass range was a hyperbolic trap with an r0 value of 2.5 mm, as reported by R. E. Kaiser et al. in Int. J. of Mass Spectrometry Ion Processes 106, 79 (1997). One problem with this and other small-scale ion traps used in mass spectrometry is their limited spectral resolution. For instance, existing small-scale ion traps typically do not provide useful mass spectral resolution below 1.0-2.0 AMUs (atomic mass units). Moreover, there is a demand for even smaller ion traps, (i.e., submillimeter with r0 and/or zvalues less than 1.0 mm), for use in mass spectrometry, though ion traps of this size exacerbate the present limitations in mass spectral resolution.
Thus, there was a need for a submillimeter ion trap with improved spectral resolution in performing mass spectrometry.
The present invention concerns a submillimeter ion trap for mass spectrometric chemical analysis. In the preferred embodiment, the ion trap is a submillimeter trap having a cavity with: 1) an effective length 2z0 with z0 less than 1.0 mm; 2) an effective radius r0 less than 1.0 mm; and 3) a z0/r0 ratio greater than 0.83. Testing demonstrates that a z0/r0 ratio in this range improves mass spectral resolution from a prior limit of approximately 1.0-2.0 AMUs, down to 0.2 AMUs, the result of which is a smaller ion trap with improved mass spectral resolution. Employing smaller ion traps without sacrificing mass spectral resolution opens a wide variety of new applications for mass spectrometric chemical analysis.
The ion trap comprises: a central electrode having an aperture; a pair of insulators, each having an aperture; a pair of end cap electrodes, each having an aperture; a first electronic signal source coupled to the central electrode; and a second electronic signal source coupled to the end cap electrodes. In the preferred embodiment, the central electrode, insulators, and end cap electrodes are united in a sandwich construction where their respective apertures are coaxially aligned and symmetric about an axis to form a partially enclosed cavity having an effective radius r0 and an effective length 2z0. Moreover, r0 and/or z0 are less than 1.0 mm, and the ratio z0/r0 is greater than 0.83.